This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

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$Id: wiring.c 248 2007-02-03 15:36:30Z mellis $*/

#include "wiring_private.h"#include "pins_arduino.h"

/* Measures the length (in microseconds) of a pulse on the pin; state is HIGH * or LOW, the type of pulse to measure. Works on pulses from 2-3 microseconds * to 3 minutes in length, but must be called at least a few dozen microseconds * before the start of the pulse. */unsigned long pulseIn(uint8_t pin, uint8_t state, unsigned long timeout){ // cache the port and bit of the pin in order to speed up the // pulse width measuring loop and achieve finer resolution. calling // digitalRead() instead yields much coarser resolution. uint8_t bit = digitalPinToBitMask(pin); uint8_t port = digitalPinToPort(pin); uint8_t stateMask = (state ? bit : 0); unsigned long width = 0; // keep initialization out of time critical area

// convert the timeout from microseconds to a number of times through // the initial loop; it takes 16 clock cycles per iteration. unsigned long numloops = 0; unsigned long maxloops = microsecondsToClockCycles(timeout) / 16;

// wait for any previous pulse to end while ((*portInputRegister(port) & bit) == stateMask) if (numloops++ == maxloops) return 0;

// convert the reading to microseconds. The loop has been determined // to be 20 clock cycles long and have about 16 clocks between the edge // and the start of the loop. There will be some error introduced by // the interrupt handlers. return clockCyclesToMicroseconds(width * 21 + 16); }